Polychloroprene composition

ABSTRACT

Crystalline polychloroprene compositions comprise mercaptan-modified or xanthogen disulfide-modified 2-chloro-1,3-butadiene homopolymers having gel contents of 5-80 percent by weight. The polymers are prepared by free radical emulsion polymerization at a temperature of 5°-20° C. to a conversion of 70-95 percent in the presence of 2.5×10 -4  -5.5×10 -4  moles of mercaptan or xanthogen disulfide modifier per mole of 2-chloro-1,3-butadiene.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation-in-part of co-pending U.S. patent applicationSer. No.08/611,114 filed Mar. 5, 1996, now abandoned which is acontinuation-in-part of application Ser. No. 08/411,183 filed Mar. 27,1995, now U.S. Pat. No. 5,527,846.

BACKGROUND OF THE INVENTION

This invention relates to polychloroprene compositions. Morespecifically, this invention relates to polychloroprene compositionswhich are particularly suited for use in aqueous adhesives employed forbonding foamed polymeric materials such as polyurethanes and polyolefinsto substrates including bonded fibre board, thermoplastic olefins,fabric, and polyurethanes.

Polymers of chloroprene (i.e. 2-chloro-1,3-butadiene) are well known,commercially available elastomers useful in the manufacture of generalrubber goods, adhesives, and coating compositions. They are especiallysuited for formulation of contact adhesives, which are generally sold inthe form of cements, i.e. solutions of polymers and additives in organicsolvents. Most commonly, the polychloroprenes used as elastomericcomponents in these cements are chloroprene homopolymers havingrelatively high levels of crystallinity. Such polymers promote rapidadhesive bond strength development, i.e. green bond strength, withoutthe requirement of a further curing step. See R. M. Murray, et al., TheNeoprenes, Elastomer Chemicals Department, E. I. du Pont de Nemours &Co. (1963), p.83.

A disadvantage of the use of highly crystalline chloroprenehomopolymers, however, is that cements containing them are susceptibleto bond failure at temperatures of, for example, 80° C.-95° C., unless asecond curing step is employed. It is known that chloroprene copolymersgenerally exhibit excellent high temperature resistance. Despite thisenhanced thermal resistance, merely substituting chloroprene copolymersfor chloroprene homopolymers is not an acceptable means to solve theproblem of temperature susceptibility of polychloroprene homopolymercements.

This is because chloroprene copolymers have inherently low crystallinityand therefore, adhesives containing these polymers exhibit inferiorgreen bond strength. Thus, neither chloroprene homopolymer cements ofthe prior art nor chloroprene copolymer cements of the prior art combineoutstanding high temperature resistance and green bond strength.

A further disadvantage of polychloroprene cements is that they usuallycontain large quantities of volatile organic solvents. Due toenvironmental concerns, and in view of the recognized safety hazardsassociated with organic solvents, the use of aqueous latex adhesives ismore desirable than the use of solvent-based adhesives in manyapplications. There is thus a need in the art for improvedpolychloroprene adhesives which exhibit superior high temperatureresistance coupled with green bond strength, yet which areenvironmentally safe.

SUMMARY OF THE INVENTION

The present invention is directed to an elastomer which comprises amercaptan-modified or xanthogen disulfide-modified crystalline2-chloro-1,3-butadiene homopolymer having a gel content of 5-80 percentby weight which is prepared by free radical emulsion polymerization at atemperature of 5° C.-20° C., to a conversion of 70-95 percent, in thepresence of 2.5×10⁻⁴ -5.5×10⁻⁴ moles of mercaptan or xanthogen disulfidemodifier per mole of 2-chloro-1,3-butadiene.

DETAILED DESCRIPTION OF THE INVENTION

The elastomers of the present invention are highly crystallinehomopolymers of chloroprene (2-chloro-1,3-butadiene). Thesepolychloroprenes are further characterized by having a gel content of5-80 percent by weight. The combination of specific polymer structureand gel content of the homopolymer permits attainment of excellent heatresistance and outstanding bond strengths when the polymer is formulatedinto aqueous latex adhesive compositions. In addition, such adhesivecompositions are environmentally acceptable because they do not requirethe presence of any organic solvent.

The polychloroprenes may be prepared by free radical-initiated emulsionpolymerization processes such as described in U.S. Pat. No. 2,567,117.That is, the emulsion polymerization is generally carried out at achloroprene monomer concentration of about 40-55 percent and at a pH of10 to 14 in the presence of a free radical generator. Examples ofsuitable free radical generators include organic or inorganic peroxides,persulfates, or hydroperoxides in a redox system, for example, in thepresence of potassium sulfite or sodium hydrosulfite. An importantfeature of the present invention is that the polymers are prepared atpolymerization temperatures of from 5° C.-20° C., preferably from 10°C.-20° C. When polymers prepared within this temperature range areformulated into adhesives, it has been found that adhesive green bondstrength is significantly enhanced.

Another principal feature of the present invention concerns the gelcontent of the chloroprene homopolymer component of the adhesive. Theterm "gel content", when used in reference to polymers, indicates theamount of polymer insoluble in an organic solvent, generally benzene,toluene, or tetrahydrofuran. As molecular weight of a polymer isincreased, a point is reached at which a crosslinked, gelled fraction isformed. Gel measurement is, therefore, a gauge of crosslinked content.The chloroprene homopolymers of the present invention have gel contentsof 5-80 percent. Polymers having gel contents of 15-35% are preferred.If polymers having gel contents lower than 5 percent are employed, hightemperature resistance is compromised, whereas if polymers having gelcontents above 80 percent are used, bonding tack and open time ofadhesives formulated from the polymers are reduced.

In the present invention, the polychloroprene gel content is based onthe amount of tetrahydrofuran-insoluble polymer present as determinedaccording to the following method:

Approximately 2 ml of latex (W), having solids content (L) is weighedand injected into a vessel containing 100 ml of tetrahydrofuran. Thevessel is rolled for not less than 30 minutes and not more than 120minutes to dissolve the soluble polymer. A 40 ml aliquot of the mixtureis then centrifuged for 1 hour to separate the insoluble gel polymerfrom the tetrahydrofuran solution. A 20 ml portion of the supernatantliquid is removed and the solvent is evaporated. The weight of thesoluble polymer (A) is then measured and the gel content is calculatedaccording to the following equation:

    % gel=100-100 (F×A)/(W×L)!

where

F=normalization factor of 5.1

A=dried solids content of soluble portion

W=weight of latex sample

L=latex solids content

The gel content of a polymer will vary depending on a number of factorsrelated to polymerization and isolation conditions. For example, theamount of gel formed increases as temperature of polymerization and/ormonomer conversion increases, whereas the amount of gel formed tends todecrease with increasing concentration of modifier, i.e. chain transferagent, present in the polymerization mixture. Gel content of thechloroprene homopolymer component may thus be controlled duringpolymerization by adjusting the polymerization temperature, amount ofchain transfer agent present, and percent monomer conversion.

Chain transfer agents are commonly used in many polymerization processesto control molecular weight without significantly alteringpolymerization kinetics. These agents react with growing polymer chainends, resulting in termination and end-capping. Consequently, thegreater the concentration of chain transfer agent, the lower will be themolecular weight of the polymer product. Polymerization in the presenceof chain transfer agents results in conditions wherein growth ofindividual polymer molecules is controlled by end-capping withouthampering kinetic chain propagation. For purposes of the presentinvention, the polymerization is carried out in the presence of a chaintransfer agent selected from the group consisting of dialkyl xanthogendisulfide modifying agent, dialkoxy xanthogen disulfide modifyingagents, or mercaptan modifying agents.

When xanthogen disulfide modifiers are used, they may be selected fromthe group consisting of dialkyl xanthogen disulfides or dialkoxyxanthogen disulfides. The dialkyl xanthogen disulfides which may be usedcan be represented by the structure: ##STR1## in which R and R' arealkyl groups containing from one to eight carbon atoms. Examples ofsuitable alkyl radicals are methyl, ethyl, propyl, isopropyl, and thevarious isomeric butyl, amyl, hexyl, heptyl, and octyl radicals. Thepreferred alkyl groups are those having one to four carbon atoms. Ofthese, diisopropyl xanthogen disulfide (PXD) is most preferred.

The dialkoxy xanthogen disulfide modifiers are compositions of theformula: ##STR2## wherein R and R' are alkyl radicals having one toeight carbon atoms and m is two to six. Examples of suitable alkylradicals are methyl, ethyl, butyl and hexyl.

Mercaptan-modified homopolymers may also be utilized to provideadhesives having characteristics equally attractive to those of thexanthogen disulfide-modified homopolymers. The long chain alkylmercaptans used as modifiers (i.e. chain transfer agents) may containfrom about four to about twenty carbon atoms and may be primary,secondary or tertiary, straight or branched, although the straight chainprimary alkyl mercaptans are usually preferred. Representativemercaptans include butyl, hexyl, octyl, dodecyl and tridecyl mercaptans,as well as mixtures of mercaptans derived from coconut oil.Dodecylmercaptan (DDM) is the preferred chain transfer agent for avariety of reasons. In general, it is the least expensive chain transferagent and the easiest to obtain. Moreover, the molecular weight of DDMis lower than that of other common chain transfer agents. Consequently,lower quantities can be used to achieve a desired gel content.

To obtain chloroprene homopolymers having gel contents within the rangeuseful in the present invention, it is most effective to regulate theamount of chain transfer agent utilized during polymerization and alsoto control monomer conversion. At conversions of approximately 70percent, chloroprene polymers begin to form relatively higher numbers ofcrosslinks due to interpolymer transfer reactions. When a chain transferagent is present in the polymerization mixture, a balance is establishedbetween interpolymer crosslinks and end-capping. Adjustment of monomerconversion within the range of 70-100 percent serves to alter themolecular weight characteristics of the polymer, and consequently, thegel content. It has been found that use of between 2.5×10⁻⁴ -5.5×10⁻⁴moles of chain transfer agent per mole of chloroprene monomer in apolymerization carried to 85 percent monomer conversion will provide gelcontents of 10 to 50 percent. Slight adjustments in the amount of chaintransfer agent or monomer conversion will provide higher or lower gelcontents as required. Generally, the useful range of chain transferagent for chloroprene polymerization is between 1×10⁻⁵ to 1×10⁻³ molesof chain transfer agent per mole of chloroprene monomer.

In preparing the chloroprene polymers useful in the present invention,polymerization of chloroprene monomer is preferably carried to aconversion of at most about 95 percent, at which point it is stopped bythe addition of a polymerization inhibitor, e.g. phenothiazine. Thepolymerization emulsion is then stabilized by the addition of an agentsuch as, e.g. the sodium salt of disproportionated abietic acid.Conversions in the range of 70-95 percent are acceptable, with thepreferred range being 80-85 percent. Conversions lower than 95 percentresult in reduced chain grafting, thereby providing an optimumcrystalline structure.

After polymerization is stopped, unpolymerized monomers are removed. Thelatex may optionally be creamed at this point to raise the solidscontent, e.g. to 55-60 percent. The creaming process causes limitedparticle agglomeration and a separation of concentrated latex from aserum phase which is very dilute in polymer.

The polymer compositions of the present invention are useful ascomponents of aqueous latex adhesives. In particular, they areespecially suited for use as polymeric components in adhesives employedfor bonding foamed polymers to various substrates. Adhesives containingthe polymers of the present invention are particularly useful inassembly of foamed cushions used in automobile interiors. Assembly ofthese parts requires adhesives having excellent performance attemperatures in excess of 120° C. because the bond must withstandambient temperatures of 100° C.-120° C. which are generated insideautomobiles parked in locations exposed to the summer sun.

Preferred latex adhesive formulations for foam bonding comprise thepolychloroprenes of the present invention, 1 to 75 parts by weight ofrosin per 100 parts by weight of polychloroprene, and a sufficientamount of water to provide a composition having a solids content of25-65 weight percent based on the weight of the polychloroprene. Therosins suitable for use include for example, acids or terpenoid esterssuch as esters of abietic acid, hydrogenated abietic acid,disproportionated abietic acid, or polymerized abietic acid. Normally,the esters are lower alkyl esters of two to six carbon atoms, but thisis not a critical limitation. Also, mixtures of rosins can be used.Rosins modify adhesive characteristics, e.g. tack, adhesion, cohesion,and hot bond strength. Adhesives of the present invention containingrosin concentrations of 1 to 75 parts per 100 parts chloroprenehomopolymer exhibit rapid bond strength and high temperature resistance.Poor adhesion to some surfaces results if levels of less than 1 partrosin is used. In contrast, if greater than 75 parts are used, the heatresistance of the compositions is decreased. Preferably 10-60 partsrosin are used per 100 parts of chloroprene homopolymer. Most preferably20-40 parts per 100 parts of chloroprene homopolymer are used becausethis provides the best balance of properties. The preferred aqueouslatexes also contain sufficient water to provide a solids content of thehomopolymer of between 25 and 65 weight percent, preferably between30-50 weight percent. Latexes having solids contents of less than 25weight percent are expensive to ship. If the solids content exceeds 65weight percent the latex is generally unstable and difficult to pump,transport, or compound without coagulation.

In order to prevent corrosion or deterioration of certain substrates,for example, metal surfaces or fabric surfaces with which the adhesivecomes in contact, it may be desirable to include an effective amount ofantioxidant in the composition. Antioxidant is generally present in thecomposition in amounts up to 5 parts by weight per 100 parts by weightof the chloroprene polymer. For this purpose, many antioxidants wellknown in the art as being useful in polychloroprene formulations areacceptable. An example of such an antioxidant is4,4'-thiobis(6-tertiary-butyl-3-methylphenol).

Various other conventional ingredients that may additionally be presentin the latexes of the present invention include curatives such as epoxycompounds and metal oxides, stabilizers, surfactants, such as sodiumlauryl sulfate, and fillers and pigments, such as carbon black, titaniumdioxide, and clay. Various resins may also be added to further modifybonding characteristics. Such resins include alkyl phenolic resins,terpene phenolic resins, hydrogenated resin esters, coumarone indeneresins, and hydrocarbon resins such as poly-alpha-methyl styrene.

Additionally, suitable acid acceptors are frequently added to the latex.Zinc oxide, which may be present as both acid acceptor and curative, mayoptionally be used in the compositions of the present invention. Acidacceptors should preferably have a surface area of at least 2 m² /gbecause such materials can be easily maintained in suspension. Zincoxide is the preferred acid acceptor because of the superior long termcure properties of the resulting homopolymer.

The preferred aqueous latex adhesives may be prepared by mixingchloroprene homopolymer latex with emulsified rosin. The solids contentof the aqueous latex adhesive may then be adjusted to between 25 and 65weight percent if it is outside this range after addition of the rosin.

The aqueous latex adhesives are then applied to appropriate substratesin an convenient manner, for example, by painting or spraying on oneside of the substrate or by roller coating. In practice, adhesives areapplied to both surfaces to be bonded and the bonding operation iscarried out within a temperature range of about 20° C. to 100° C. Theadhesives exhibit outstanding heat resistance and green bond strength.For example, bonds which can maintain their integrity when used to bondfoam in a strained curve configuration can be formed within seconds ofapplication. These bonds do not fail when heated to 140° C. and exhibitbond strengths above the tear strength of the foam substrate itself. Inaddition, the adhesives resist hot soapy water soaks of 45 minutes withno loss of bond strength.

EXAMPLES Test Methods

Physical properties of polymers prepared in the following examples weretested according to the following procedures.

Lap Shear

Samples of Wilsonart® brand decorative high pressure laminates(available from Ralph Wilson Plastics, Inc.) measuring 2.54 cm×10.16cm×1.27 mm were bonded to samples of particle board measuring 2.54cm×10.16 cm×1.27 cm and coated with 20-30g/m² dry weight of an adhesivein a manner such that there was a 5.08 cm overlap along the 10.16 cmsurface. After 24 hours, bonded samples, measuring 2.54 cm×15.24 cm,were removed and tested as follows. Each sample was mounted verticallyin a tensile test machine and pulled at a uniform rate of gripseparation of 5.08 cm per minute at room temperature. The peak stress orthe force required to separate the pieces completely was reported asfailure force in units of kPa. The test was repeated at a temperature of80° C.

Cleavage

Sample specimens were prepared as described in the lap shear test. Eachspecimen was mounted horizontally in a 73° C. oven by suspending thesample by the nonoverlapped end of the particle board, particle boardside facing upward. A 500 g weight was attached to the non-overlappedend of the laminate and the time at which complete adhesive failureoccurred, as indicated by complete separation of the adhesive bond, wasreported as the average of three tests.

Creep

Canvas strips (0.05 g/cm² unbleached cotton duck) measuring 11.18cm×2.54 cm were coated with a layer of adhesive sufficient to cover thefabric, leaving 2.54 cm at one end uncoated. A second coat of adhesivewas applied and the fabric was dried. The strips were bonded together,adhesive faces together, congruent without overlap, and the uncoatedends in contact, by passing the strips through laminating rollers at apressure of 137.9 kPa and a linear transport speed of 1 meter/minute.The sample was placed in an oven at 80° C. in a "T-peel" configuration.That is, the uncoated section of one layer of the laminate was clampedvertically to a rigid support mounted in a vertical position in the ovenand the uncoated section of the facing layer of the sample was loadedwith a 500 g weight. The length of opened bond was reported incentimeters at intervals of 15 minutes. Adhesive bonds which failedcompletely were designated with an "F".

Peel

Canvas to canvas test specimens were prepared and suspended in an ovenas in the above-described creep test. Temperature was continuouslyincreased at a rate of 2° C. every 5 minutes from an initial temperatureof 40° C. to a final temperature of 180° C. The failure temperature wasrecorded as that temperature at which the adhesive bond completelyseparated. If the sample remained intact, 180° C. was shown as thetemperature resistance.

Example 1

An aqueous emulsion was prepared by emulsifying 100 parts chloroprene,0.07 parts of dodecylmercaptan (3.1×10⁻⁴ moles per mole of chloroprene),2.5 parts of disproportionated abietic acid, 0.9 parts of the potassiumsalt of a napthalenesulfonic acid formaldehyde condensate, 0.15 partsdimer acid, stabilized with 0.0001 part of p-tert-butylcatechol, and 80parts water. Polymerization was carried out under nitrogen at 9° C. inthe presence of a redox/free radical initiator system consisting of 0.1part dextrose, 0.2 parts sodium formaldehyde sulfoxylate, 0.2 partstrisodium phosphate, 0.1 part potassium sulfite, and 0.001 parts sodium2-anthraquinonesulfonate. A 5% solution of potassium persulfate wasadded dropwise until a conversion of 85% was attained. Thepolymerization was stopped by addition of 0.04 parts phenothiazine andthe aqueous dispersion was stabilized with 0.44 parts of the sodium saltof disproportionated abietic acid. Unpolymerized monomer was removed bysteam distillation at reduced pressure. The resultant chloroprenehomopolymer latex contained 20% gel polymer insoluble intetrahydrofuran. An adhesive latex was then prepared by mixing thecomponents shown in Table I with the chloroprene homopolymer latex. Theproportions in Table I are stated in amounts of additive per 100 partsof polychloroprene in the latex. Properties of adhesive bonds preparedusing the adhesive latex are shown in Table I.

Example 2

A chloroprene homopolymer latex was prepared substantially as describedin Example 1, using the same reaction conditions, reaction temperature,and amounts of reactants, catalysts and other ingredients except that0.12 parts of diisopropylxanthogen disulfide per 100 parts ofchloroprene (3.9×10⁻⁴ moles per mole of chloroprene) was added as achain transfer agent in place of dodecylmercaptan. After removal ofunreacted monomer, the resultant latex had a gel content of 12.2%. Anadhesive latex was then prepared by mixing the components shown in TableI with the chloroprene homopolymer latex. The proportions in Table I arestated in amounts of additive per 100 parts of polychloroprene.

Control Example A

A chloroprene homopolymer latex was prepared substantially as describedin Example 1 using the same conditions and amounts of reactants,catalysts, and other ingredients except that the polymerization tookplace at a temperature of 14° C. and the amount of dodecylmercaptanpresent was 0.09 parts per 100 parts chloroprene monomer (3.9×10⁻⁴ molesper mole of chloroprene). The gel content of the resultant latex afterremoval of unreacted monomer was 4.6%. An adhesive latex was thenprepared by mixing the components shown in Table I with the chloroprenehomopolymer latex. The proportions in Table I are stated in amounts ofadditive per 100 parts of polychloroprene. Properties of adhesive bondsprepared using the adhesive latex are shown in Table I.

Control Example B

A chloroprene homopolymer latex was prepared substantially as describedin Example 1 using the same conditions and amounts of reactants,catalysts, and other ingredients except that the polymerization tookplace at a temperature of 9° C. and 0.11 parts of dodecylmercaptan per100 parts chloroprene monomer (4.8×10⁻⁴ moles per mole ofchloroprene)was used in place of 0.07 part dodecylmercaptan. The gelcontent of the resultant latex after removal of unreacted monomer was1.1%. An adhesive latex was then prepared by mixing the components shownin Table I with the chloroprene homopolymer latex. The proportions inTable I are stated in amounts of additive per 100 parts ofpolychloroprene. Properties of adhesive bonds prepared using theadhesive latex are shown in Table I.

                  TABLE I                                                         ______________________________________                                                                       Control                                                                              Control                                                                Example                                                                              Example                                 Adhesive Formulation                                                                       Example 1                                                                              Example 2                                                                              A      B                                       ______________________________________                                        Chloroprene  100      100      100    100                                     Rosin        30       30       30     30                                      Zinc Oxide   4        4        4      4                                       Antioxidant.sup.1                                                                          2        2        2      2                                       % Solids     49       51       58     50                                      Polymer Gel Content (%)                                                                    20       12.2     4.6    1.1                                     Conversion (%)                                                                             85       92       NA     86                                      Lap Shear @ 22° C.                                                                  1682     1372     1358   1455                                    (kPa)                                                                         Lap Shear @ 80° C.                                                                  331      359      124    79                                      (kPa)                                                                         Cleavage Test                                                                              14:23    14:45    3:43   1:09                                    (minutes:seconds to                                                           failure; average of 3                                                         tests)                                                                        Creep Test (cm)                                                               Bonds Aged 1 week                                                             15 minutes   0.762    0.508    3.048  11.176                                  30 minutes   1.016    0.762    6.858  .sup. F.sup.2                           45 minutes   1.016    0.762    11.176 F                                       60 minutes   1.016    0.762    F      F                                       120 minutes  1.270    0.762    F      F                                       Peel Test (°C.).sup.3                                                               175      180      89     73                                      ______________________________________                                         .sup.1 4,4thiobis(6-tertiary-butyl-3-methylphenol)                            .sup.2 Bond failure                                                           .sup.3 Temperature resistance test                                       

Example 3

Two chloroprene homopolymers, A and B, were prepared according to themethod described in Example 1 using dodecylmercaptan as a chain transferagent to adjust gel content. All conditions, amounts of catalyst, andother ingredients were the same as in Example 1, except as noted inTable II. Components listed in Table II are in parts per 100 partsmonomer. Polymer samples A and B were used to prepare adhesive latexesfalling within the scope of the present invention according to themethod described in Example 1. Results of peel tests using the adhesiveformulations are also shown in Table II.

                  TABLE II                                                        ______________________________________                                        Example 3            A       B                                                ______________________________________                                        Polymerization Temperature (°C.)                                                            10      10                                               Chloroprene Monomer  100     100                                              Dodecylmercaptan     0.08.sup.1                                                                            0.09.sup.2                                       Disproportionated abietic acid                                                                     2.2     2.4                                              Sodium alkylaryl sulfonate                                                                         0       1                                                Dextrose             0.1     0.1                                              p-tert-Butylcatechol 0       0                                                Sodium 2-anthraquinonesulfonate                                                                    0       0                                                Conversion (%)       87.9    91                                               Gel Content (%)      5.6     11.2                                             Peel Test (°C.)                                                                             164     174                                              Temperature Resistance                                                        ______________________________________                                         .sup.1 3.5 × 10.sup.-4 moles per mole of chloroprene                    .sup.2 4.6 × 10.sup.-4 moles per mole of chloroprene               

Example 4

Three chloroprene homopolymers were prepared according to the methoddescribed in Example 1 using diisopropylxanthogen disulfide as a chaintransfer agent to adjust gel content. All conditions, amounts ofcatalyst and other ingredients were the same, except as noted in TableIII. Components listed in Table III are in parts per 100 parts monomer.Polymer samples A-C were used to prepare adhesive latexes falling withinthe scope of the present invention according to the method described inExample 1. Results of peel tests using the adhesive formulations arealso shown in Table III.

                  TABLE III                                                       ______________________________________                                        Example 4         A        B        C                                         ______________________________________                                        Polymerization Temperature (°C.)                                                         11       11       10                                        Chloroprene Monomer                                                                             100      100      100                                       Diisopropylxanthogen disulfide                                                                  0.16.sup.1                                                                             0.1.sup.2                                                                              0.12.sup.3                                Disproportionated abietic acid                                                                  2.2      2.4      2.4                                       Dextrose          0.1      0.1      0.1                                       p-tert-Butylcatechol                                                                            0        0        0.001                                     Sodium 2-anthraquinonesulfonate                                                                 0        0        0                                         Conversion (%)    88.3     85.4     91.9                                      Gel Content (%)   78.7     45.1     12.2                                      Peel Test (°C.)                                                                          165      155      180                                       Temperature Resistance                                                        ______________________________________                                         .sup.1 5.2 × 10.sup.-4 moles per mole of chloroprene                    .sup.2 3.3 × 10.sup.-4 moles per mole of chloroprene                    .sup.3 3.9 × 10.sup.-4 moles per mole of chloroprene               

We claim:
 1. An elastomer composition which comprises amercaptan-modified or xanthogen disulfide-modified crystalline2-chloro-1,3-butadiene homopolymer having a gel content of 5-80 percentby weight which is prepared by free radical emulsion polymerization at atemperature of 5° C.-20° C., to a conversion of 70-95 percent, in thepresence of 2.5×10⁻⁴ -5.5×10⁻⁴ moles of mercaptan or xanthogen disulfidemodifier per mole of 2-chloro-1,3-butadiene.
 2. The composition of claim1 wherein the gel content of the 2-chloro-1,3-butadiene homopolymer is15-35 percent by weight.
 3. The composition of claim 1 wherein thetemperature of the emulsion polymerization is 10°-15° C.
 4. Thecomposition of claim 1 wherein the 2-chloro-1,3-butadiene homopolymer isa dodecylmercaptan-modified 2-chloro-1,3-butadiene homopolymer.
 5. Thecomposition of claim 1 wherein the 2-chloro-1,3-butadiene homopolymer isa diisopropylxanthogen disulfide-modified 2-chloro-1,3-butadienehomopolymer.
 6. The composition of claim 1 wherein the2-chloro-1,3-butadiene homopolymer is prepared by emulsionpolymerization to a monomer conversion of 80-85 percent.